Mood tense over missing gold sovereign (6)
I believe the answer is:
temper
'mood' is the definition.
(temper is one's mood or state of mind)
'tense over missing gold sovereign' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'over' means one lot of letters go next to another.
'missing gold' becomes 'emp' (I am not sure about this - if you are sure you should believe this answer much more).
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
't'+'emp'+'er'='TEMPER'
(Other definitions for temper that I've seen before include "Modify" , "Paddy - moderate" , "Irritability" , "Bad humour" , "Display of anger" .)